Why do some people refuse to be happy?

My mom asked me to move here in another state and I did, partially to make her happy & partially because I was bored in my situation before. So when I got there, she attacked me in the car out of nowhere. A week later I confronted her asking why she got mad and another violent fight went on and she was threatening to kill herself which made no logic sense I just asked her why she did what she did a week ago. She threatened to kill me. So idk why she has to act all angry all the time and my dad and I seem capable of love and peace and everything, having love within ourselves and respect for other people. But she on the other hand like deflects it. We even asked her why, & we say she can feel good, it's okay, all that. But she was just rejecting the idea. She always sems sad & miserable all the time. My mom I wish I could take all her sadness away. But she like does it to herself. I mean like days after days seeing her always acting or putting on this mask like she's miserable (she's, sad to say, always been like this) it's almost like you think it's your fault, it's because of you, but she's like this everywhere she goes. She also causes some of the stuff like I kind of don't trust her because she attacked me in the car that one time and I don't deserve it. I have to hold my own self respect to a point and I'm not going to just let someone abuse me. But I mean she makes her own bed & lies in it, why doesn't she not do things on a continuous basis that will cause people to react/act the way they do. I mean it's like karma/reprocussions catches up with her, bad things follow if you do bad things, why doesn't she see this or care???